To build up a masonry wall it is necessary to lay individual blocks by a hand operation one by one in a bed of mortar and to adjust each block in turn so that its position is in alignment and properly spaced from the next proceeding block together with the course of block beneath it. For a workman to keep the blocks level and of unfirm thickness, he had to rely on visual alignment and measurement which often produced an unprofessional looking result as there was formerly no convenient tool for this purpose.